1. Technical Field
The present invention generally relates to methods and apparatus for adapting transmit parameters to radio conditions in a wireless communications system, and particularly relates to adapting mobile terminal transmit parameters, including CDMA spreading parameters, in order to reduce the energy consumption and extend the battery life of the mobile terminal.
2. Background
The design of the uplink (or reverse link) signal transmitted from the mobile terminal to a base station, or “Node B”, presents difficult challenges in advanced cellular systems such as Wideband Code Division Multiple Access (W-CDMA) systems. The demand for higher data rates drives the design of more advanced modulation schemes. These advanced modulation schemes, in turn, create new challenges for the design of cost-effective, energy-efficient portable devices. In particular, these advanced uplink designs tend to increase power consumption at the mobile terminal and place heavier demands on the batteries of portable devices.
A key component of a mobile terminal's power consumption is the power consumed by the terminal's transmitter power amplifier (PA). In order to maximize the efficiency of the power amplifier, the uplink signal format should have a low Peak-to-Average-Power Ratio (PAPR). The best case, in terms of power amplifier efficiency, is a constant-envelope modulation, in which the information resides only in the phase of the carrier. A constant-envelope scheme permits the use of non-linear (class C) PAs, which can be operated with high efficiency. The widespread GSM system, for example, uses a modulation scheme of this type.
However, more advanced modulation schemes, where information also resides in the amplitude of the modulated carrier, are increasingly used to increase data rates and more efficiently utilize limited spectrum resources. These modulation schemes cause higher PAPRs on the transmitted signals, and thus put higher demands on the PAs. In particular, the PAs must meet more stringent linearity requirements, and thus tend to operate less efficiently. Accordingly, for a given transmit power, more power is consumed and more energy is drained from the mobile terminal's battery.
W-CDMA systems use spread spectrum technology, where a channel is defined by a spreading code. In first generation W-CDMA terminals, a single channelization code is used, and the uplink signal is formed by combining an in-phase (I) and quadrature (Q) branch, each modulated with BPSK. The resulting PAPR is acceptable, but the peak data rate is limited to 384 kb/s. In the second generation, called High Speed Packet Access (HSPA), up to four subchannels with corresponding spreading codes are used, but low-order phase modulation is still used. To achieve the highest peak data rates, up to 5.76 Mb/s, very low spreading factors are used. In the current development of enhancements to HSPA (so-called HSPA+), higher-order modulation formats, such as 16-QAM, are being considered. In addition, more than four subchannels may be used. Finally, multiple-input multiple-output (MIMO) antenna techniques are also being standardized. These measures will increase the peak rate, but generally at the expense of increased PAPR. Therefore, power consumption and battery life in future mobile terminals continue to be challenged.